1. Technical Field
The present invention is directed generally toward laser diodes, and more particularly to a grating-outcoupled surface emitting laser with an integrated high speed modulation.
2. Description of the Related Art
Transmission of light through waveguides has been pursued for many types of communications applications. Light signals offer many potential advantages over electronic signals. Light sources are commonly-created from semiconductor devices, and include semiconductor devices such as light emitting diodes (LED) and laser diodes (LD).
Optical fiber is the most commonly used transmission medium for light signals. A single fiber is capable of carrying several different modulated signals within it at one time. For instance, wavelength division multiplexing divides the used bandwidth of the fiber into different channels, each channel containing a small range of wavelengths, and thus transmits several different wavelengths (or signals) of light at once. Using such a system requires sources for the different wavelengths. More wavelengths on the fiber require more sources to be coupled to the fiber.
Efficient coupling of light into a fiber is simplified if the laser beam has a cross sectional profile that matches the profile of the fiber mode(s). Efficient use of light for communications requires that the light have high temporal coherence. Efficient coupling of light to monomode guides requires spatial coherence. Spatial coherence requires the laser to operate in a single lateral and transverse mode. Temporal coherence requires the laser to operate in a single longitudinal mode and implies a very narrow bandwidth, or range of wavelengths.
The most coherent semiconductor lasers use resonators based on grating feedback rather than Fabry-Perot resonators with reflective end facets. Distributed feedback (DFB) lasers use a Bragg reflective grating covering the entire pumped length of the laser. An alternative to DFB lasers is the use of distributed Bragg reflectors (DBRs) located outside the pumped region.
The Grating-Outcoupled Surface-Emitting (GSE) laser (described in commonly assigned U.S. patent application Ser. Nos. 09/844,484 and 09/845,029, both of which are hereby incorporated by reference), is an essentially planar structure that provides out-of-plane optical emission. The GSE laser has a built in horizontal waveguide that allows on-wafer or on-chip routing and control of light along with emission from the surface of the wafer or chip. In contrast, the light from vertical cavity surface-emitting lasers (VCSELs) is directed normal to the wafer or chip surface and cannot easily be routed within the wafer or chip. The epitaxial structure of a VCSEL is very thick and therefore costly and time consuming to grow, compared to the relatively thin layers making up an edge-emitting (EE) or GSE laser. While EE lasers have a horizontal waveguide and can route light within a wafer or chip, at least one terminating edge (cleaved or etched) is required to access or connect the on-chip light to the outside world. Thus EE lasers are inherently edge-bound (and hence not fully integrable), while VCSELs have incompatibility due to their very special epitaxy requirements.
When the laser is turned on and begins emitting light, photons are introduced into the lasing cavity by applying a current to the lasing cavity. The amount of light, or power, supplied by the laser must ramp up as the density of photons increases in the lasing cavity. Similarly, as the laser is turned off, the amount of power supplied by the laser decreases as the density of photons in the lasing cavity decreases. This process of turning the laser on and off takes time and affects the speed of communication for the laser.